The present invention relates to starting systems for AC induction motors, and is particularly directed to an improved speed-sensing motor start control, especially one employed for controlling AC current that is supplied to the start winding of an AC induction motor. It should be noted here that motor speed and motor current are inversely proportional; as such is the case, current sensing can be used as a mechanism to characterize motor speed.
It is well known that AC single-phase induction motors require some sort of starting mechanism to rotate the magnetic field of the field windings, so as to generate sufficient torque to start the rotor. The starting mechanism enables the rotor to overcome the static forces associated with accelerating the rotor and load, for example, from a zero initial angular velocity.
Typically, single-phase induction motors are equipped with at least two sets of windings: a run winding for driving the rotor at normal speed and an auxiliary or start winding to generate the required starting torque. In order to provide the necessary rotating field for the starter, a phase-shifting device, typically a capacitor, is connected in series with the start winding. During start up of the motor both the run and auxiliary windings are utilized to bring the motor up to a sufficient high speed. Thereafter, the start or auxiliary winding is disconnected, so that the motor will operate on the run winding only for more efficient operation. In the event that a heavy load is encountered and the motor rpm decreases significantly, the auxiliary winding may be cut in to add torque to overcome the increased load.
In most AC induction motors, the structure of the auxiliary winding is such that sustained connection to the AC line voltage would overheat and possibly destroy the start windings, if not the entire motor. For the start winding circuits in split-phase motors, the start capacitor is also susceptible to damage if the auxiliary or start winding is not disconnected after the motor has achieved its designed cut out rpm. In other types of multiple winding motors, the shaft may not accelerate completely to its designed full speed until the auxiliary or start winding cuts out. Therefore, an efficient and cost-effective control circuit is necessary for automatically connecting and disconnecting the auxiliary or start windings at the appropriate times during and after start under and high load conditions.
An ideal motor starting control device should have the following desirable characteristics:
A. The starting device should disconnect the start auxiliary winding at a predetermined speed, independent of actual line voltage, load, and temperature.
B. Whenever the motor shaft load exceeds the motor torque and a breakdown is exceeded, the device should cut in the start auxiliary winding prior to the stalling of the motor.
C. The device should have long life and high reliability, even in the presence of line voltage surges.
D. The device should cut in and cut out the start winding without requiring adjustment, with either connection with a dual-voltage motor or with a capacitor start or other split-phase motor.
E. Alignment with the shaft or rotor should not be required. More specifically, the control element may be remotely installed and independent of the physical location of the motor.
F. A single calibration should satisfy all required ratings.
G. The device should be capable of operation at different voltages, including high voltages (i.e., above 240 vac).
H. The device should be subject to adjustment via a simple means, such as a potentionmeter.
Until recently, the start circuit for controlling the current to the motor start windings has taken the form either of a mechanical centrifugal switch, located on the armature or shaft, or an electromechanical current-sensing relay device. Because of the arcing and wear problems associated with motor switching currents, both the centrifugal switch and the current-sensing relay have proved to have rather short lives and do not meet the reliability criteria mentioned above.
Centrifugal switch systems, however, do have the advantages of voltage and temperature independence, low initial cost, and a switching action which is dependent entirely on the rotation speed of the motor shaft, regardless of the load. Also, upon heavy loading and significantly decreased motor speed, the centrifugal switch will reconnect or cut in the start winding. The main drawback of the centrifugal switch is that, because it utilizes a set of mechanical contacts, it is susceptible to pitting and arcing, and the contacts will eventually fail. A corrosive, humid or dusty environment will accelerate the contact failure. Centrifugal switches have a life expectancy typically much less than 1,000,000 operations at full load. In the event of centrifugal switch failure the contacts often weld themselves closed, thus sending continuous AC current through the start capacitor and start windings, possibly destroying these elements. Centrifugal switches are not readily field replaceable or adjustable as they are located internally within the motor housing. A further disadvantage is related to unreliable switching levels with age due to metal ratigue in the activating mechanism.
The current-sensing relay type device has the advantages of low cost, temperature independence, and permitting the start winding to start or cut in at a predetermined speed dependent only on current. This device, however, does utilize mechanical contacts and has a rather short life expectancy for the same reasons as the centrifugal switch. In addition, this type of system has a positional dependency associated with its mounting, as it relies on gravity, at least in part, to open the switch contacts. Other drawbacks include noise due to contact movement and the possibility that the relay contacts stick or remain closed during high current surges. This type of failure engages the start winding for an indefinite prolonged period of time thus damaging both the start winding and the start capacitor. An improved type of motor starting circuit, which employs a reed/triac combination, has been proposed, for example, in Fink et al U.S. Pat. No. 3,766,457. This type of motor starting circuit is a low cost, temperature independent approach, and cuts in or cuts out the start windings based on field current, which increases with decreased motor speed. This type of circuit also has a relatively long life, as compared to the above mentioned centrifugal switch and current-sensing relay, because the heavy start current load is handled through a solid-state power triac. The overall performance of this device is rather good; however, it does have several drawbacks.
One drawback is that it requires heavy current-carrying conductor wire to be wrapped around the reed switch in order to generate a specific magnetic field to effect reed closure. Heavy duty motors often have field windings of fourteen gage wire or heavier, and it is difficult to effect the proper number of turns of this heavy wire around the rather miniscule reed switch bulb to generate the specific magnetic field necessary for reed switch closure for a particular induction motor. Also, the reed switch contacts open and close twice during each cycle of the alternating current, and considering that a start up may have a duration of ten seconds, there will be 1,200 reed contact transfers effected for each start up operation. Consequently, the device life is quite load dependent. As with any device with moving parts, the reed switch will eventually arc and wear and will finally freeze into an open or closed state. Finally it is required that the reed switch bulb be located where it will be insulated from chattering or vibration, which could damage this device or reinsert the start windings undesirably.
Another disadvantage of reed switches is the effect of extraneous magnetic fields sometimes present in the same environment which will effect operation of the reed switch thus altering motor performance. Further, reed switches offer a major drawback in manufacturing whereby selection of individual reeds with a certain ampere-turns is required for proper operation of the motor reed switch combination.
An attempt at a completely solid-state motor starting circuit was proposed in Lewus U.S. Pat. No. 3,916,275. This motor starting control circuit fulfills nearly all of the ideal characteristics mentioned previously, most significantly a long lifetime due to the absence of mechanical parts or contacts. However, there are several significant weaknesses to this motor starting control circuit.
The circuit of the Lewus Patent relies on the facts a. that a certain voltage will develop across a sensing element in series with the run windings, and b. that this voltage can be rectified and applied between the gate and main terminal #1 of a triac to gate the same. This system employs a current sense resistor to develop a voltage in excess of the trigger diodes to the gate of the triac. The circuit operation is very much dependent on the gate sensitivity of the triac. During starting in rush, significant voltage must be present across the sense resistor to trigger the triac and enable the start windings; which may not occur if the input line voltage is low. Conversely, an input line voltage that is higher than nominal will cause excessive voltage and current to be present at the triac gate during in rush, possibly destroying the triac. Because the system is so dependent on triac gate sensivity, calibration is difficult as a result of the device dependency. Most importantly, however, the system is dependent on supply voltage, unlike the present invention.
Another desirable feature for semiconductor-control starting circuits is the option to stack triacs or other equivalent power switching devices, especially for higher voltage applications (i.e., with a motor voltage above 240 VAC). However, with the Lewus triggering scheme, the power triacs may not be stacked; that would only be possible if the circuit that senses the current through the sense resistors were somehow electrically isolated from the triac. Only by triggering a triac from its gate to its second main terminal can the triac be stacked or combined. An example of this type of stacking, with reference to the reed-triac type of motor control circuit, is shown in Miller U.S. Pat. No. 4,463,304. Other proposed motor start control circuits have included timer-controlled semiconductor switches to connect the start circuitry for a prescribed amount of time. This approach, while avoiding some of the drawbacks of the above-mentioned devices, does not rely on motor speed at all and thus cannot sense an overload condition; therefore, it may cut out start winding prematurely upon start up. Because it is a timed device, the control cannot permit reinsertion of start winding in the event of heavy loading after start up. Another type of start control circuit employs a positive temperature coefficient resistor to disable the start windings after a start up. This system is both temperature sensitive and very slow to reset because the positive temperature coefficient element must cool down completely before it can cut in the start winding.